Until now, bismuth-molybdenum mixed oxides have been obtained in the state of the art by precipitation methods, sol-gel methods or solid-state reactions.
US 2007/0238904 A1 discloses a bismuth-molybdenum mixed oxide which is obtained by precipitation and subsequent calcining. The bismuth-molybdenum mixed oxide is suitable as catalyst for the conversion of propylene or isobutylene to acrolein or methacrolein.
WO 2008/028681 and DE 10 2006 032 452 A1 disclose a method for the production of nanocrystalline metal oxides or mixed metal oxides. There is no indication in these documents that special nanocrystalline bismuth-molybdenum mixed oxides which are particularly suitable as catalyst for the conversion of for example propylene to acrolein can be produced with the method.
A crystalline molybdenum mixed oxide and thus also a bismuth-molybdenum mixed oxide can be obtained only with difficulty via conventional methods. Thus, G. A. Zenkovets et al., “The structural genesis of a complex (MoVW)5O14 oxide during thermal treatments and its redox behaviour at elevated temperatures”, Materials Chemistry and Physics, 103 (2007), 295-304, disclose that a molybdenum mixed oxide obtained via precipitation and spray drying has an amorphous structure. This mixed oxide is present in the form of large aggregates approximately 5 μm in size. A partially nanocrystalline structure forms inside the aggregates due to subsequent calcining. A pure crystalline phase with crystallites more than 1000 nm in size forms only after prolonged thermal treatment at approximately 440° C. The production of a nanocrystalline molybdenum mixed oxide can thus be accomplished only with difficulty, in particular with regard to the production of small crystallites.
A disadvantage of the lead-molybdenum mixed oxides of the state of the art is thus in particular that a uniform particle size of the molybdenum mixed oxides cannot be obtained and a control of the crystallization, in particular with regard to the crystallite size, is not possible. The BET surface area of the molybdenum mixed oxides of the state of the art is likewise mostly too small. A small particle size with as large as possible a BET surface area is desired, in particular for catalytic uses.